Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. In particular, a common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in a database to determine proper identification of a person, such as for verification purposes.
A typical electronic fingerprint sensor is based upon illuminating the finger surface using visible light, infrared light, or ultrasonic radiation. The reflected energy is captured with some form of camera, for example, and the resulting image is framed, digitized and stored as a static digital image. For example, U.S. Pat. No. 4,210,899 to Swonger et al. discloses an optical scanning fingerprint reader cooperating with a central processing station for a secure access application, such as admitting a person to a location or providing access to a computer terminal. U.S. Pat. No. 4,525,859 to Bowles similarly discloses a video camera for capturing a fingerprint image and uses the minutiae of the fingerprints, that is, the branches and endings of the fingerprint ridges, to determine a match with a database of reference fingerprints.
Unfortunately, optical sensing may be affected by stained fingers or an optical sensor may be deceived by presentation of a photograph or printed image of a fingerprint rather than a true live fingerprint. In addition, optical schemes may require relatively large spacings between the finger contact surface and associated imaging components. Moreover, such sensors typically require precise alignment and complex scanning of optical beams. Accordingly, optical sensors may thus be bulky and be susceptible to shock, vibration and surface contamination. Accordingly, an optical fingerprint sensor may be unreliable in service in addition to being bulky and relatively expensive due to optics and moving parts.
U.S. Pat. No. 4,353,056 to Tsikos discloses another approach to sensing a live fingerprint. In particular, the patent discloses an array of extremely small capacitors located in a plane parallel to the sensing surface of the device. When a finger touches the sensing surface and deforms the surface, a voltage distribution in a series connection of the capacitors may change. The voltages on each of the capacitors is determined by multiplexor techniques. Unfortunately, the resilient materials required for the sensor may suffer from long term reliability problems. In addition, multiplexing techniques for driving and scanning each of the individual capacitors may be relatively slow and cumbersome. Moreover, noise and stray capacitances may adversely affect the plurality of relatively small and closely spaced capacitors.
Significant advances have been made in the area of integrated circuit fingerprint sensor, as disclosed, for example, in U.S. Pat. Nos. 5,828,773 and 5,862,248, both assigned to the assignee of the present invention. The disclosed sensors are based upon generating an electric field which can sense the ridges of a fingerprint despite contamination, skin surface damage, and other factors. The sensor is relatively compact and rugged. The sensing die may be mounted on a leadframe so that the conductive pins extend outwardly from side edges of the package.
As disclosed in U.S. Pat. No. 5,862,248 to Salatino et al., an electrically conductive ring may be formed surrounding an opening in the encapsulating package. The integrated circuit die is exposed through the opening. The conductive ring may be used to drive the finger of the user to produce the fingerprint image from a plurality of sensing electrodes on the surface of the integrated circuit. Electrical contact between the conductive ring and the integrated circuit may be established by positioning the ring on an insulating layer on the die and forming one or more conductor filled vias through the insulating layer.
U.S. Pat. No. 5,940,526 to Setlak et al. discloses additional advances in the area of electric field fingerprint sensors. In particular, two electrically conductive rings are provided on the exterior upper surface of the housing. One ring may be used to discharge accumulated electrical charge from the finger of the user. The other electrode can be used to sense finger contact to thereby wake-up the device and supply power thereto. Connections from the integrated circuit die to the external rings may be made by conductor filled vias extending through the housing.
The disclosed sensor embodiments of U.S. Pat. No. 5,862,248, for example, are traditional flat IC packages wherein the integrated circuit die is carried by a leadframe. Pins of the leadframe extend outwardly from the encapsulating plastic housing for connection to a printed circuit board. Unfortunately for many applications, it may not be inconvenient to connect the sensor directly onto a conventional motherboard, for example. This is especially the case in a laptop computer, for example, where direct connection to a motherboard is not practical, and available mounting space is at a premium. Yet another disadvantage with the conventional packaging is that the finger of the user may be uncomfortably bent backwards to permit the fingertip to be positioned on the flat sensor surface.